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SubjectRe: [PATCH 3/3 RFC paul/rcu/srcu] srcu: flip only once for every grace period
On 02/22/2012 05:29 PM, Lai Jiangshan wrote:
>>From 4ddf62aaf2c4ebe6b9d4a1c596e8b43a678f1f0d Mon Sep 17 00:00:00 2001
> From: Lai Jiangshan <laijs@cn.fujitsu.com>
> Date: Wed, 22 Feb 2012 14:12:02 +0800
> Subject: [PATCH 3/3 RFC paul/rcu/srcu] srcu: flip only once for every grace period
>
> flip_idx_and_wait() is not changed, and is split as two functions
> and only a short comments is added for smp_mb() E.
>
> __synchronize_srcu() use a different algorithm for "leak" readers.
> detail is in the comments of the patch.
>
> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> ---
> kernel/srcu.c | 105 ++++++++++++++++++++++++++++++++++----------------------
> 1 files changed, 64 insertions(+), 41 deletions(-)
>
> diff --git a/kernel/srcu.c b/kernel/srcu.c
> index a51ac48..346f9d7 100644
> --- a/kernel/srcu.c
> +++ b/kernel/srcu.c
> @@ -249,6 +249,37 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
> */
> #define SYNCHRONIZE_SRCU_READER_DELAY 5
>
> +static void wait_idx(struct srcu_struct *sp, int idx, bool expedited)
> +{
> + int trycount = 0;

Hi, Paul

smp_mb() D also needs to be moved here, could you fix it before push it.
I thought it(smp_mb()) always here in my mind, wrong assumption.

Sorry.

Thanks,
Lai

> +
> + /*
> + * SRCU read-side critical sections are normally short, so wait
> + * a small amount of time before possibly blocking.
> + */
> + if (!srcu_readers_active_idx_check(sp, idx)) {
> + udelay(SYNCHRONIZE_SRCU_READER_DELAY);
> + while (!srcu_readers_active_idx_check(sp, idx)) {
> + if (expedited && ++ trycount < 10)
> + udelay(SYNCHRONIZE_SRCU_READER_DELAY);
> + else
> + schedule_timeout_interruptible(1);
> + }
> + }
> +
> + /*
> + * The following smp_mb() E pairs with srcu_read_unlock()'s
> + * smp_mb C to ensure that if srcu_readers_active_idx_check()
> + * sees srcu_read_unlock()'s counter decrement, then any
> + * of the current task's subsequent code will happen after
> + * that SRCU read-side critical section.
> + *
> + * It also ensures the order between the above waiting and
> + * the next flipping.
> + */
> + smp_mb(); /* E */
> +}
> +
> /*
> * Flip the readers' index by incrementing ->completed, then wait
> * until there are no more readers using the counters referenced by
> @@ -258,12 +289,12 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
> * Of course, it is possible that a reader might be delayed for the
> * full duration of flip_idx_and_wait() between fetching the
> * index and incrementing its counter. This possibility is handled
> - * by __synchronize_srcu() invoking flip_idx_and_wait() twice.
> + * by the next __synchronize_srcu() invoking wait_idx() for such readers
> + * before start a new grace perioad.
> */
> static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
> {
> int idx;
> - int trycount = 0;
>
> idx = sp->completed++ & 0x1;
>
> @@ -278,28 +309,7 @@ static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
> */
> smp_mb(); /* D */
>
> - /*
> - * SRCU read-side critical sections are normally short, so wait
> - * a small amount of time before possibly blocking.
> - */
> - if (!srcu_readers_active_idx_check(sp, idx)) {
> - udelay(SYNCHRONIZE_SRCU_READER_DELAY);
> - while (!srcu_readers_active_idx_check(sp, idx)) {
> - if (expedited && ++ trycount < 10)
> - udelay(SYNCHRONIZE_SRCU_READER_DELAY);
> - else
> - schedule_timeout_interruptible(1);
> - }
> - }
> -
> - /*
> - * The following smp_mb() E pairs with srcu_read_unlock()'s
> - * smp_mb C to ensure that if srcu_readers_active_idx_check()
> - * sees srcu_read_unlock()'s counter decrement, then any
> - * of the current task's subsequent code will happen after
> - * that SRCU read-side critical section.
> - */
> - smp_mb(); /* E */
> + wait_idx(sp, idx, expedited);
> }
>
> /*
> @@ -307,8 +317,6 @@ static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
> */
> static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
> {
> - int idx = 0;
> -
> rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
> !lock_is_held(&rcu_bh_lock_map) &&
> !lock_is_held(&rcu_lock_map) &&
> @@ -318,27 +326,42 @@ static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
> mutex_lock(&sp->mutex);
>
> /*
> - * If there were no helpers, then we need to do two flips of
> - * the index. The first flip is required if there are any
> - * outstanding SRCU readers even if there are no new readers
> - * running concurrently with the first counter flip.
> - *
> - * The second flip is required when a new reader picks up
> + * When in the previous grace perioad, if a reader picks up
> * the old value of the index, but does not increment its
> * counter until after its counters is summed/rechecked by
> - * srcu_readers_active_idx_check(). In this case, the current SRCU
> + * srcu_readers_active_idx_check(). In this case, the previous SRCU
> * grace period would be OK because the SRCU read-side critical
> - * section started after this SRCU grace period started, so the
> + * section started after the SRCU grace period started, so the
> * grace period is not required to wait for the reader.
> *
> - * However, the next SRCU grace period would be waiting for the
> - * other set of counters to go to zero, and therefore would not
> - * wait for the reader, which would be very bad. To avoid this
> - * bad scenario, we flip and wait twice, clearing out both sets
> - * of counters.
> + * However, such leftover readers affect this new SRCU grace period.
> + * So we have to wait for such readers. This wait_idx() should be
> + * considerred as the wait_idx() in the flip_idx_and_wait() of
> + * the previous grace perioad except that it is for leftover readers
> + * started before this synchronize_srcu(). So when it returns,
> + * there is no leftover readers that starts before this grace period.
> + *
> + * If there are some leftover readers that do not increment its
> + * counter until after its counters is summed/rechecked by
> + * srcu_readers_active_idx_check(), In this case, this SRCU
> + * grace period would be OK as above comments says. We defines
> + * such readers as leftover-leftover readers, we consider these
> + * readers fteched index of (sp->completed + 1), it means they
> + * are considerred as exactly the same as the readers after this
> + * grace period.
> + *
> + * wait_idx() is expected very fast, because leftover readers
> + * are unlikely produced.
> */
> - for (; idx < 2; idx++)
> - flip_idx_and_wait(sp, expedited);
> + wait_idx(sp, (sp->completed - 1) & 0x1, expedited);
> +
> + /*
> + * Starts a new grace period, this flip is required if there are
> + * any outstanding SRCU readers even if there are no new readers
> + * running concurrently with the counter flip.
> + */
> + flip_idx_and_wait(sp, expedited);
> +
> mutex_unlock(&sp->mutex);
> }
>



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